Carbon dioxide electrolysis device and method of operating carbon dioxide electrolysis device

ABSTRACT

A carbon dioxide electrolysis device includes: a cathode configured to reduce carbon dioxide and thus form a carbon compound; an anode configured to oxidize water and thus generate oxygen; a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide; an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and a separator provided between the anode and the cathode. An aspect ratio of the cathode gas flow path is greater than 1 and 3 or less. In a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M&lt;h/4.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2021-044811, filed on Mar. 18, 2021; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments relate to a carbon dioxide electrolysis device.

BACKGROUND

In recent years, renewable energy such as solar power is desirablyconverted into not only electrical energy for use hut also a storableand transportable resource in terms of both energy and environmentalissues. This demand has advanced research and development of artificialphotosynthesis technology, which uses sunlight to produce chemicalsubstances like photosynthesis in plants. This technology has potentialto store the renewable energy as storable fuel and is also expected tocreate value by producing chemical substances that can be used asindustrial raw materials.

Known examples of a device that uses the renewable energy such as solarpower to produce chemical substances, include an electrochemicalreaction device having a cathode and an anode, the cathode beingconfigured to reduce carbon dioxide (CO₂) generated from a power plantor waste treatment plant, and the anode being configured to oxidizewater (H₂O). The cathode, for example, reduce carbon dioxide to producecarbon compounds such as carbon monoxide (CO). When such anelectrochemical reaction device is fabricated in a cell form (alsocalled an electrolysis cell), it may be effective to fabricate it in aform similar to a fuel cell, such as a polymer electric fuel cell(PEFC). The directly supply of carbon dioxide to a catalyst layer of thecathode, rapidly processes a carbon dioxide reduction reaction.

However, such a cell form has a problem similar to that of the PEFCarises. In other words, it is necessary to stably supply carbon dioxideto the cathode catalyst layer to fabricate the electrolysis cell that isresistant to failure and durable and to improve efficiency of carboncompound production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram to explain a configuration example of acarbon dioxide electrolysis device.

FIG. 2 is a planar schematic diagram illustrating a structural exampleof a part of a flow path plate.

FIG. 3 is a cross-sectional schematic diagram illustrating a structuralexample of a part of the flow path plate.

FIG. 4 is a planar schematic diagram illustrating another structuralexample of the flow path plate.

FIG. 5 is a cross-sectional schematic diagram illustrating anotherstructural example of a part of the flow path plate.

FIG. 6 is a cross-sectional schematic diagram illustrating still anotherstructural example of a part of the flow path plate.

FIG. 7 is a cross-sectional schematic diagram illustrating yet anotherstructural example of a part of the flow path plate.

FIG. 8 is a schematic diagram illustrating another configuration exampleof the carbon dioxide electrolysis device.

FIG. 9 is a flowchart to explain an operating method example of thecarbon dioxide electrolysis device.

FIG. 10 is a flowchart to explain an operation example of a refreshoperation step.

FIG. 11 is a diagram illustrating a relationship between a partialcurrent density of carbon monoxide and a utilization ratio of carbondioxide at a cathode.

FIG. 12 is a diagram illustrating a relationship between the partialcurrent density of carbon monoxide and the utilization ratio of carbondioxide at the cathode.

DETAILED DESCRIPTION

A carbon dioxide electrolysis device includes: a cathode configured toreduce carbon dioxide and thus form a carbon compound; an anodeconfigured to oxidize water and thus generate oxygen; a cathode gas flowpath facing on the cathode and configured to supply gas containingcarbon dioxide; an anode solution flow path facing on the anode andconfigured to supply an electrolytic solution containing water; and aseparator provided between the anode and the cathode. An aspect ratio ofthe cathode gas flow path is greater than 1 and 3 or less, the aspectratio being defined by a ratio of a depth of the cathode gas flow pathto a width of the cathode gas flow path. In a cross-section along adirection perpendicular to a facing surface between the cathode and thecathode gas flow path in the cathode gas flow path, a fluid mean depth Mof the cathode gas flow path and a depth h of the cathode gas flow pathsatisfy a formula: h/8≤M<h/4, the fluid mean depth M being defined by aratio of a circumferential length of the cathode gas flow path to across-sectional area of the cathode gas flow path.

Hereinafter, embodiments will be described with reference to thedrawings. The drawings are schematic, and dimensions such as a thicknessand a width of each component are sometimes different from actual ones.In each embodiment presented below, substantially the same componentsare denoted by the same reference signs, and a description thereof issometimes partially omitted.

In this specification, “connection” includes not only direct connectionbut also indirect connection, unless otherwise specified.

FIG. 1 is a schematic diagram to explain a configuration example of acarbon dioxide electrolysis device. FIG. 1 illustrates a carbon dioxideelectrolysis device 1 including an electrolysis cell 10.

The electrolysis cell 10 includes an anode part 11, a cathode part 12,and a separator 13 that separates the anode part 11 from the cathodepart 12. The electrolysis cell 10 is, for example, sandwiched between apair of support plates and further tightened with bolts or the like.

The anode part 11 includes an anode 111, an anode solution flow path 112a provided on a flow path plate 112, and an anode current collector 113.

The cathode part 12 includes a cathode 121, a cathode gas flow path 122a provided on a flow path plate 122, and a cathode current collector123.

The anode 111 is an electrode (oxidation electrode) that promotes anoxidation reaction of water (H₂O) in an anode solution to produce oxygen(O₂) and hydrogen ions (H⁺), or an oxidation reaction of hydroxide ions(OH⁻) generated in the cathode part 12 to produce oxygen and water.

The anode 111 is disposed between the separator 13 and the flow pathplate 112 to be in contact therewith. A first surface of the anode 111is in contact with the separator 13. A second surface of the anode 111is provided on an opposite side of the first surface of the anode 111and faces the anode solution flow path 112 a.

Compounds produced by the oxidation reaction of the anode 111 aredifferent depending on types of oxidation catalysts and other factors.When an electrolytic solution is used as the anode solution, the anode111 is preferably mainly composed of a catalyst material (anode catalystmaterial) capable of oxidizing water (H₂O) to produce oxygen andhydrogen ions, or oxidizing hydroxide ions (OH⁻) to produce water andoxygen, and capable of decreasing an overvoltage of such reactions. Suchcatalyst materials include metals such as platinum (Pt), palladium (Pd),and nickel (Ni), alloys and intermetallic compounds containing thesemetals, binary metal oxides such as manganese oxide (Mn—O), iridiumoxide (Ir—O), nickel oxide (Ni—O), cobalt oxide (Co—O), iron oxide(Fe—O), tin oxide (Sn—O), indium oxide (In—O), ruthenium oxide (Ru—O),lithium oxide (Li—O), and lanthanum oxide (La—O), ternary metal oxidessuch as Ni—Co—O, Ni—Fe—O, La—Co—O, Ni—La—O, and Sr—Fe—O, quaternarymetal oxides such as Pb—Ru—Ir—O and La—Sr—Co—O, and metal complexes suchas Ru complexes and Fe complexes.

The anode 111 is preferably equipped with a substrate (support) having astructure that enables a movement of the anode solution and ions betweenthe separator 13 and the anode solution flow path 112 a, such as a meshmaterial, a punching material, or a porous structure such as a porousmember. The substrate having the porous structure also includes thesubstrate with relatively large pores, such as a metal fiber sinteredcompact. The substrate may be composed of metals such as titanium (Ti),nickel (Ni), and iron (Fe) or a metal material such as an alloycontaining at least one of these metals (for example, SUS), or may becomposed of the anode catalyst materials described above. When oxidesare used as the anode catalyst material, a catalyst layer is preferablyformed by attaching or laminating the anode catalyst material to asurface of the substrate made of the metal material described above. Theanode catalyst material preferably has a shape such as nanoparticles,nanostructures, nanowires to enhance the oxidation reaction. Thenanostructures are structures with nanoscale irregularities formed on asurface of the catalyst material. The oxidation catalyst does notnecessarily have to be provided on the oxidation electrode. An oxidationcatalyst layer provided other than the oxidation electrode may beelectrically connected to the oxidation electrode.

The cathode 121 is an electrode (reduction electrode) that generatesreduction reactions of carbon dioxide and reduction products to producecarbon compounds. Examples of the carbon compounds include carbonmonoxide (CO), formic acid (HCOOH), methane (CH₄), ethane (C₂H₆),ethylene (C₂H₄), methanol (CH₃OH), acetic acid (CH₃COOH), ethanol(C₂H₅OH)), formaldehyde (HCHO), propanol (C₃H₇OH), and ethylene glycol(C₂H₆O₂). Along with the reduction reaction of carbon dioxide, thereduction reaction at the cathode 121 may include a side reaction thatgenerates a water reduction reaction to produce hydrogen (H₂).

The cathode 121 is preferably composed of an ion-conductive substance inaddition to an electrode substrate and a metal catalyst supported on acarbon material. The ion-conductive substance exhibits an action oftransferring ions between metal catalysts contained in layers and thusexerts effects of enhancing the electrode activity. A cation exchangeresin or anion exchange resin is preferably used as the aboveion-conductive substance.

The support for the metal catalyst preferably has the porous structure.In addition to the above materials, applicable materials include, forexample, carbon blacks such as Ketjen black and Vulcan XC-72, activatedcarbon, carbon nanotubes, and the like. By having the porous structure,an area of an active surface that contributes to the oxidation-reductionreaction can be increased, thus increasing conversion efficiency.

The catalyst layer itself formed on the substrate as well as the supportpreferably has the porous structure and has a large number of relativelylarge pores. Specifically, in terms of a pore size distribution of thecatalyst layer measured by a mercury intrusion method, a frequency ofpore distribution is preferably maximized in a range of 5 μm or more and200 μm or less in diameter. In this case, gas diffuses quicklythroughout the catalyst layer, and reduction products are easilydischarged outside the catalyst layer through this pathway, resulting inan efficient electrode.

A gas diffusion layer is preferably provided on the electrode substratesupporting the catalyst layer to efficiently supply carbon dioxide tothe catalyst layer. The gas diffusion layer is formed by a conductiveporous member. The gas diffusion layer is preferably formed by awater-repellent porous member because amounts of water produced by thereduction reaction and water that has moved from an oxidation side canbe lowered, allowing the water to drain through a reduction flow pathand increasing the percentage of carbon dioxide gas in the porousmember.

When a thickness of the gas diffusion layer is extremely small,uniformity on a cell surface is impaired, which is not desirable. On theother hand, when the thickness is extremely large, a member costincreases and efficiency decreases due to the increase in gas diffusionresistance, which is not desirable. A denser diffusion layer (mesoporouslayer (MPL)) is more preferably provided between the gas diffusion layerand the catalyst layer to further improve diffusibility, as it changesthe water repellency and porosity to promote gas diffusibility andliquid component drainage.

The metal catalysts supported on the above support include materialsthat decrease activation energy for the reduction of hydrogen ions andcarbon dioxide. In other words, metal materials that decrease theovervoltage in the reduction reaction of carbon dioxide to producecarbon compounds are included. For example, it is preferred to use atleast one metal selected from the group consisting of gold (Au), silver(Ag), copper (Cu), platinum (Pt), palladium (Pd), nickel (Ni), (Co),iron (Fe), manganese (Mn), titanium (Ti), cadmium (Cd), zinc (Zn),indium (In), gallium (Ga), lead (Pb), and tin (Sn), and metal oxide oralloys containing these metals. For example, at least one of copper,gold, and silver is preferably used. For example, metal complexes suchas ruthenium (Ru) complexes or rhenium (Re) complexes, can also be usedas reduction catalysts without being limited to the above. A pluralityof materials may also be mixed. Various shapes can be applied to themetal catalyst, such as plate, mesh, wire, particle, porous, thin-film,and island shapes.

When metal nanoparticles are applied to the metal catalyst, an averagediameter is preferably 1 nm or more and 15 nm or less, more preferably 1nm or more and 10 nm or less, and even more preferably 1 nm or more and5 nm or less. Meeting these conditions is desirable because a surfacearea of the metal per catalyst weight becomes larger, and a small amountof metal is required to exhibit high activity.

The anode 111 and cathode 121 can be connected to a power supply 20. Thepower supply 20 applies a voltage between the anode 111 and cathode 121.Examples of the power supply 20 are not limited to ordinary system powersupplies or batteries but may include power sources that supply powergenerated by renewable energy sources such as solar cells or wind power.The power supply 20 may further include a power controller that adjustsoutput of the above power supply to control the voltage between theanode 111 and cathode 121. The power supply 20 may be provided outsidethe carbon dioxide electrolysis device 1.

The anode solution flow path 112 a has a function of supplying the anodesolution to the anode 111. The anode solution flow path 112 a is formedby pits (grooves/recesses) provided in the flow path plate 112. The flowpath plate 112 has an inlet and an outlet port (both not illustrated)connected to the anode solution flow path 112 a, and the anode solutionis introduced and discharged by a pump (not illustrated) through theseinlet and outlet ports. The anode solution is distributed in the anodesolution flow path 112 a to be in contact with the anode.

An aqueous solution containing metal ions (electrolytic solution) can beused as the anode solution. By using the electrolytic solutioncontaining metal ions, the electrolysis efficiency can be increased. Theaqueous solution can be, for example, aqueous solutions containingphosphate ions (PO₄ ²⁻), borate ions (BO₃ ³⁻), sodium ions (Na⁺),potassium ions (K⁺), calcium ions (Ca²⁺), lithium ions (Li⁺), cesiumions (Cs⁺), magnesium ions (Mg²⁺), chloride ions (Cl⁻), hydrogencarbonate ions (HCO₃ ⁻), carbonate ions (CO₃ ²⁻), and other aqueoussolutions. Other aqueous solutions containing LiHCO₃, NaHCO₃, KHCO₃,CsHCO₃, phosphoric acid, boric acid, and so on may also be used.

The cathode gas flow path 122 a faces a first surface of the cathode121. The cathode gas flow path 122 a has a function of supplying gascontaining carbon dioxide to the cathode 121. For example, the cathodegas flow path 122 a can be connected to a carbon dioxide supply sourcethat supplies the gas containing carbon dioxide. The carbon dioxidesupply source can be, for example, a power plant, waste treatment plant,or other facilities. The cathode gas flow path 122 a is formed by pits(grooves/recesses) provided in the flow path plate 122. The flow pathplate 122 has an inlet and an outlet port (both not illustrated)connected to the cathode gas flow path 122 a, and the gas is introducedand discharged by a pump (not illustrated) through these inlet andoutlet ports.

It is preferable to use materials for the flow path plates 112 and 122that have low chemical reactivity and high electrical conductivity.Examples of such materials include, for example, metal materials such asTi and SUS, carbon, and the like. The flow path plates 112 and 122 havethe inlet and outlet ports for each flow path, as well as screw holesfor tightening, although these are not illustrated in the drawing.Packing, not illustrated, is sandwiched between a front and back of eachflow path plate as necessary. Although the flow path plates 112 and 122are mainly formed from a single member, they may also be formed fromdifferent members and laminated together. Furthermore, hydrophilic andwater-repellent functions may be added by applying surface treatment topart or all thereof.

The flow path plate 122 can have a land in contact with the cathode 121for electrical connection with the cathode 121. A shape of the cathodegas flow path 122 a can be adjacent to a columnar land, a serpentineshape where a thin flow path is folded or the like, but any shape with acavity can be used. The cathode gas flow path 122 a is preferablyconstituted by a plurality of flow paths connected in parallel, aserpentine flow path, or a combination thereof because the uniformity ofthe gas to be supplied to the cathode 121 can be enhanced and theuniformity of an electrolytic reaction can be enhanced.

FIG. 2 is a planar schematic diagram illustrating a structural exampleof a part of the flow path plate 122. FIG. 2 illustrates an X-Y plane ofthe flow path plate 122 including an X-axis and a Y-axis orthogonal tothe X-axis. In FIG. 2, only a superposition of the flow path plate 122and the cathode 121 is schematically illustrated. FIG. 3 is across-sectional schematic diagram illustrating a structural example of apart of the flow path plate 122. FIG. 3 illustrates a Y-Z plane of theflow path plate 122 including the Y-axis and a Z-axis orthogonal to theY-axis and X-axis. The Z-axis direction is a thickness direction of theflow path plate 122.

The flow path plate 122 has a surface 241, a surface 242, and thecathode gas flow path 122 a. The surface 241 is in contact with thecathode 121. The surface 242 is provided on an opposite side of thesurface 241 and in contact with the cathode current collector 123. Theflow path plate 122 illustrated in FIG. 2 and FIG. 3 has a rectangularparallelepiped shape. A three-dimensional shape of the flow path plate122 is not limited to the rectangular parallelepiped shape.

The cathode gas flow path 122 a faces the gas diffusion layer of thecathode 121. The cathode gas flow path 122 a is connected to the inletand outlet ports. The inlet port is provided for introducing the gascontaining carbon dioxide into the cathode gas flow path 122 a. Theoutlet port is provided for discharging the gas containing carbondioxide from the cathode gas flow path 122 a and for dischargingproducts of the reduction reaction from the cathode gas flow path 122 a.

The cathode gas flow path 122 a illustrated in FIG. 2 extends in aserpentine shape along the surface 241. The cathode gas flow path 122 amay also extend in a comb-teeth or spiral shape along the surface 241without being limited to the serpentine shape. The cathode gas flow path122 a includes a space formed by, for example, a groove or openingprovided at the flow path plate 122.

The carbon dioxide gas may be supplied in a dry state. A carbon dioxideconcentration in the gas supplied to the cathode gas flow path 122 adoes not have to be 100%. In this case, it is also possible to reducethe gas containing carbon dioxide emitted from each of variousfacilities, although the efficiency will decrease.

The flow path plates 112 and 122 preferably have the same shape as eachother. This can improve uniformity of the reaction. The flow path plates112 and 122 may have different shapes from each other.

The anode current collector 113 is in contact with a surface of the flowpath plate 112 opposite to a contact surface with the anode 111. Theanode current collector 113 is electrically connected to the anode 111.The anode current collector 113 preferably contains a material with lowchemical reactivity and high electrical conductivity. Such materialsinclude metal materials such as Ti and SUS, carbon, and the like.

The cathode current collector 123 is in contact with a surface of theflow path plate 122 opposite to a contact surface with the cathode 121.The cathode current collector 123 is electrically connected to thecathode 121. The cathode current collector 123 preferably contains amaterial with low chemical reactivity and high electrical conductivity.Such materials include metal materials such as Ti and SUS, carbon, andthe like.

The separator 13 is provided between the anode 111 and the cathode 121.The separator 13 is formed of an ion-exchange membrane or the like thatcan move ions between the anode and the cathode and can separate theanode part from the cathode part. For example, cation-exchange membranessuch as Nation and Fremion, and anion-exchange membranes such asNeosepta, Selemion, and Sustenion can be used as the ion-exchangemembrane. When an alkaline solution is used as the electrolytic solutionand the moving of mainly hydroxide ions (OH⁻) is mainly assumed, theseparator is preferably formed of the anion-exchange membrane. Theion-exchange membrane may be formed by using a membrane with ahydrocarbon basic structure or a membrane with an amine group. Inaddition to the ion-exchange membrane, salt bridges, glass filters,porous polymer membranes, porous insulating materials, and so on may beapplied to the separator as long as the material is capable of movingions between the anode and the cathode. However, when gas distributionoccurs between the cathode and anode parts, circular reactions due toreoxidation of reduction products may occur. Therefore, it is preferableto have less gas exchange between the cathode and anode parts.Therefore, care should be taken when using a porous thin membrane as theseparator.

Next, an operation example of the carbon dioxide electrolysis device ofthe embodiment will be explained. Here, the case where the carbondioxide electrolysis device 1 illustrated in FIG. 1 produces carbonmonoxide as a carbon compound is mainly explained, but the carboncompound as the reduction product of carbon dioxide is not limited tocarbon monoxide. The reduction product, carbon monoxide, may be furtherreduced to produce organic compounds as described above. Theelectrolysis cell 10 is preferably used when producing solution carboncompounds. A reaction process by the electrolysis cell 10 may be thecase mainly producing hydrogen ions (H⁺) or the case mainly producinghydroxide ions (OH⁻) but is not limited to either of these reactionprocesses.

The reaction process is described mainly for the oxidation of water(H₂O) to produce hydrogen ions (H⁺). When the current is supplied fromthe power supply 20 between the anode 111 and cathode 121, the oxidationreaction of water (H₂O) occurs at the anode 111 in contact with theanode solution. Specifically, as shown in Equation (1) below, H₂Ocontained in the anode solution is oxidized to produce oxygen (O₂) andhydrogen ions (H⁺).

2H₂O→4H⁺+O₂+4e   (1)

H⁺ produced at the anode 111 moves through the electrolytic solutionpresent in the anode 111 and the separator 13 to reach near the cathode121. Electrons (e⁻) in accordance with the current supplied to thecathode 121 from the power supply 20 and H⁺ that moves near the cathode121 causes the reduction reaction of carbon dioxide (CO₂). Specifically,as shown in Equation (2) below, carbon dioxide supplied to the cathode121 from the cathode gas flow path 122 a is reduced to produce carbonmonoxide. Hydrogen is also produced when hydrogen ions receiveelectrons, as shown in Equation (3) below. In this case, hydrogen may beproduced simultaneously with carbon monoxide.

CO₂+2H⁺+2e⁻→CO+H₂O   (2)

2H⁺+2e⁻→H₂   (3)

Next, a reaction process when carbon dioxide (CO₂) is mainly reduced toproduce hydroxide ions (OH⁻) is described. When the current is suppliedfrom the power supply 20 between the anode 111 and the cathode 121,water (H₂O) and carbon dioxide (CO₂) are reduced near the cathode 121 toproduce carbon monoxide (CO) and hydroxide ions (OH⁻), as shown inEquation (4) below. In addition, hydrogen is produced when waterreceives electrons as shown in Equation (5) below. At this time,hydrogen may be produced simultaneously with carbon monoxide. Thehydroxide ions (OH⁻) produced by these reactions diffuse near the anode111, and as shown in Equation (6) below, hydroxide ions (OH⁻) areoxidized to produce oxygen (O₂).

2CO₂+2H₂O+4e⁻→2CO+4OH⁻  (4)

2H₂O+2e⁻→H₂+2OH⁻  (5)

4OH⁻→2H₂O+O₂+4e⁻  (6)

In the electrolysis cell 10 illustrated in FIG. 1, the anode solutionand ions are supplied from the separator 13, and carbon dioxide gas issupplied from the cathode gas flow path 122 a.

The carbon dioxide electrolysis device 1 can not only specialize incarbon dioxide reduction, but can also produce carbon dioxide reductionproducts and hydrogen, for example, by having carbon monoxide andhydrogen in any ratio, such as 1:2, and then producing methanol in asubsequent chemical reaction.

Since hydrogen is an inexpensive and readily available raw material fromwater electrolysis and fossil fuels, it is not necessary to have a largeratio of hydrogen. From these points of view, a ratio of carbon monoxideto hydrogen of at least 1 or more, and preferably 1.5 or more, ispreferable in terms of economic efficiency and environmentalfriendliness.

The cathode gas flow path 122 a is preferably shallow in terms ofsupplying carbon dioxide to the gas diffusion layer. On the other hand,when the cathode gas flow path 122 a is narrow, pressure loss increasesin the cathode gas flow path 122 a, which is not desirable in terms ofenergy loss in the gas supply. Furthermore, when salts are precipitateddue to a reaction between metal ions in the anode solution and thecarbon dioxide gas, and the salts solidify at a boundary with the gasdiffusion layer of the cathode gas flow path 122 a, the shallow flowpath will be closed and prevent the carbon dioxide gas from spreading toan entire electrode surface, which may cause failure to affectdurability.

in one example of a conventional fuel cell, it is known that a groove isformed on an inner bottom surface of a flow path to store foreignmatters (water droplets) to prevent the flow path from being blocked bythe foreign matters (water droplets). However, in the case of the carbondioxide electrolysis device, since the precipitated salts solidify neara facing surface between the cathode 121 and the cathode gas flow path122 a, forming a groove on the inner bottom surface is not effective inpreventing the blockage.

On the other hand, in the carbon dioxide electrolysis device of thisembodiment, a cross-sectional shape of the cathode gas flow path 122 ais controlled to prevent the blockage of the flow path. An aspect ratioof the cathode gas flow path 122 a is preferably greater than 1 and 3 orless. The aspect ratio of the cathode gas flow path 122 a is defined bya ratio of a depth h of the cathode gas flow path 122 a to a width W ofthe cathode gas flow path 122 a in the X-axis or Y-axis direction.

When the aspect ratio is less than 1, the flow path may be blocked dueto the salt precipitation. When the aspect ratio exceeds 3, the flowpath plate 122 needs to be thickened, which increases material andprocessing costs. The aspect ratio is more preferably 2 or more and 3 orless.

A fluid mean depth M of the cathode gas flow path 122 a and the depth hof the cathode gas flow path 122 a preferably satisfy a formula (A)below,

A formula: h/8≤M<h/4   (A)

The fluid mean depth M of the cathode gas flow path 122 a is defined bya cross-sectional area Ac of the cathode gas flow path 122 a to acircumferential length S of the cathode gas flow path 122 a. Thecircumferential length S may be calculated by (width W×2)+(depth h×2).Even if the aspect ratio of the cathode gas flow path 122 a is large,when the fluid mean depth M of the cathode gas flow path 122 a is small,salts may precipitate in the cathode gas flow path 122 a and easilyblock the cathode gas flow path 122 a.

When the fluid mean depth M is less than h/8, the flow path may beblocked due to the salt precipitation. When the fluid mean depth M ish/4 or more, a utilization ratio of carbon dioxide may decrease. Thefluid mean depth M is more preferably h/7.9 or more and h/6 or less.

The depth h, the width W, the circumferential length S, and the fluidmean depth M can be measured by the following method. A cross-section ofthe flow path plate 122 in a perpendicular direction (X-axis directionin FIG. 2) to a long direction (Y-axis direction in FIG. 2) of thecathode gas flow path 122 a is cut out at an arbitrary position, and thecross-section is observed with a microscope, for example, to measureeach parameter. Also, as a non-destructive inspection method, forexample, neutron radiography can be used to visualize an inside of theflow path plate. It is preferable to calculate these values by averagingvalues at multiple locations.

The enough deep of the cathode gas flow path 122 a to meet the aboveconditions can form a space in a depth direction of the cathode gas flowpath 122 a where the gas can be diverted. This is desirable in terms ofeliminating blockage of gas supply during salt precipitation and makesit easier to supply the carbon dioxide gas to the entire surface of thecathode 121 even if salts precipitate at a part of the cathode gas flowpath 122 a, and is therefore less prone to failure and more desirable interms of durability. Thus, the decrease in the electrolysis efficiencycan be prevented, and the carbon dioxide electrolysis device that ishighly efficient and can be operated for a long time can be provided.

The shape of the cathode gas flow path 122 a is not limited to theshapes illustrated in FIG. 2 and FIG. 3. Other examples of the shape ofthe cathode gas flow path 122 a are described below.

FIG. 4 is a planar schematic diagram illustrating another structuralexample of the flow path plate 122. FIG. 4 illustrates the X-Y plane ofthe flow path plate 122. The flow path plate 122 illustrated in FIG. 4differs from the flow path plate 122 illustrated in FIG. 2 in that thecathode gas flow path 122 a has a plurality of flow path parts 244connected in parallel in the X-Y plane. The other parts are the same asthe flow path plate 122 illustrated in FIG. 2, so the above explanationcan be used as appropriate.

The plurality of flow path parts 244 extend along the long direction(Y-axis direction in FIG. 4) of the cathode gas flow path 122 a. FIG. 4illustrates an example of two flow path parts 244 connected in parallelfor each fold of the cathode gas flow path 122 a, but the number of flowpath parts 244 is not limited to the number illustrated in FIG. 4. Thewidth W of the cathode gas flow path 122 a illustrated in FIG. 4 isdefined by a width of one flow path part 244.

FIG. 5 is a cross-sectional schematic diagram illustrating still anotherstructural example of a part of the flow path plate 122. FIG. 5illustrates the Y-Z plane of the flow path plate 122. The flow pathplate 122 illustrated in FIG. 5 differs from the flow path plate 122illustrated in FIG. 3 in that the cathode gas flow path 122 a hasregions 122 a 1 and 122 a 2 in an X-Z cross-section. The other parts arethe same as the flow path plate 122 illustrated in FIG. 3, so the aboveexplanation can be used as appropriate.

The region 122 a 1 faces the cathode 121 and has an inner wall surface246. A cross-sectional shape of the region 122 a 1 illustrated in FIG. 5is a rectangle, but the shape of the region 122 a 1 is not limited tothe shape in FIG. 5.

The region 122 a 2 is provided between the region 122 a 1 and an innerbottom surface 245 of the cathode gas flow path 122 a and has an innerwall surface 247. A cross-sectional shape of the region 122 a 2illustrated in FIG. 5 is a rectangle, but the shape of the region 122 a2 is not limited to the shape in FIG. 5.

A width W2 in the X-axis direction of the region 122 a 2 is wider than awidth W1 in the X-axis direction of the region 122 a 1. In the flow pathplate 122 illustrated in FIG. 5, a space for the carbon dioxide gas tobe diverted can be increased, and thus the blockage of the cathode gasflow path 122 a due to the salt precipitation can be prevented by makingthe width W2 wider than the width W1. The width W of the cathode gasflow path 122 a illustrated in FIG. 5 is defined by the width W1. Thefluid mean depth M is defined by the circumferential length of thecathode gas flow path 122 a having the shape illustrated in FIG. 5,taking into account both the width W1 and the width W2.

FIG. 6 is a cross-sectional schematic diagram illustrating still anotherstructural example of a part of the flow path plate 122. FIG. 6illustrates the Y-Z plane of the flow path plate 122. Compared to theflow path plate 122 illustrated in FIG. 5, the flow path plate 122illustrated in FIG. 6 has a different shape of the region 122 a 2 in theX-Z cross-section. The other parts are the same as the flow path plate122 illustrated in FIG. 5, so the above explanation can be used asappropriate.

A cross-sectional shape of the region 122 a 2 illustrated in FIG. 6 is asquare, but the shape of the region 122 a 2 is not limited to the shapein FIG. 6. In FIG. 6, a cross-sectional area of the region 122 a 2 islarger than that of the region 122 a 1. This allows for a larger spacefor the carbon dioxide gas to be diverted and prevents the blockage ofthe cathode gas flow path 122 a due to the salt precipitation. The widthW of the cathode gas flow path 122 a illustrated in FIG. 6 is defined bythe width W1. The fluid mean depth M is defined by the circumferentiallength of the cathode gas flow path 122 a having the shape illustratedin FIG. 6, taking into account both the width W1 and the width W2.

FIG. 7 is a cross-sectional schematic diagram illustrating yet anotherstructural example of a part of the flow path plate 122. FIG. 7illustrates the Y-Z plane of the flow path plate 122. The flow pathplate 122 illustrated in FIG. 7 differs from the flow path plate 122illustrated in FIG. 3 in that the cathode gas flow path 122 a has theregions 122 a 1 and 122 a 2 in the X-Z cross-section, the inner wallsurface 246 of the region 122 a 1 is hydrophilic, and the inner wallsurface 247 and the inner bottom surface 245 of the region 122 a 2 arewater repellent. The other parts are the same as the flow path plate 122illustrated in FIG. 3, so the above explanation can be used asappropriate.

A contact angle with water at the hydrophilic inner wall surface 246 is,for example, more than 0 degrees and 90 degrees or less. The hydrophilicinner wall surface 246 can be formed using a flow path layer containing,for example, a hydrophilic material. The hydrophilic inner wall surface246 may also be formed by applying a hydrophilic treatment to the flowpath layer containing a material applicable to the flow path plate 122.

A contact angle with water at the water-repellent inner wall surface 247is, for example, 100 degrees or more and less than 180 degrees. Thewater-repellent inner wall surface 247 can be formed using a flow pathlayer containing, for example, a water-repellent material. Thewater-repellent inner wall surface 247 may also be formed by applying awater-repellent treatment to the flow path layer containing the materialapplicable to the flow path plate 122.

A thickness (length in the Z-axis direction) of the region 122 a 1 isnot particularly limited, but is preferably half the depth h of thecathode gas flow path 122 a or more, for example.

In the cathode gas flow path 122 a illustrated in FIG. 7, for example,when metal ions in the anode solution flow into the cathode gas flowpath 122 a, the anode solution tends to flow into the region 122 a 1 byforming the hydrophilic inner wall surface 246 and the water-repellentinner wall surface 247. Therefore, the salt precipitation can beprevented in the region 122 a 2, and the blockage of the cathode gasflow path 122 a due to the salt precipitation can be prevented. Thestructure illustrated in FIG. 7 may be combined with the strictureillustrated in FIG. 5 or FIG. 6 as appropriate.

Second Embodiment

FIG. 8 is a schematic diagram illustrating another configuration exampleof the carbon dioxide electrolysis device. The carbon dioxideelectrolysis device 1 illustrated in FIG. 8 includes the electrolysiscell 10, an anode solution supply system 100 that supplies an anodesolution to the electrolysis cell 10, a gas supply system 300 thatsupplies carbon dioxide (CO₂) gas to the electrolysis cell 10, a productcollection system 400 that collects products produced by a reductionreaction in the electrolysis cell 10, a control system 500 that detectstypes and amounts of the collected products as well as controls theproducts and a refresh operation, a waste solution collection system 600that collects a waste solution of the anode solution, and a refreshmaterial supply 700 that recovers the anode, cathode, and so on of theelectrolysis cell 10. Components necessary for the refresh operation donot necessarily have to be provided.

The electrolysis cell 10 corresponds to the electrolysis cell 10illustrated in FIG. 1. The explanation of each component of theelectrolysis cell 10 in the first embodiment can be used as appropriate.

In FIG. 8, the power supply 20 that applies current to the anode 111 andthe cathode 121 is provided. The power supply 20 is connected to theanode current collector 113 and the cathode current collector 123through a current introduction member. The power supply 20 is notlimited to an ordinary system power supply, batteries, and the like, butmay also have a power source that supplies power generated by renewableenergy sources such as solar cells and wind power. The power supply 20may have the above power source, a power controller that adjusts outputof the power source to control a voltage between the anode 111 and thecathode 121, or the like.

The anode solution is supplied as an electrolytic solution from theanode solution supply system 100 to the anode solution flow path 112 aof the anode part 11. The anode solution supply system 100 circulatesthe anode solution so that the anode solution is distributed in theanode solution flow path 112 a. The anode solution supply system 100 hasa pressure controller 101, an anode solution tank 102, a flow ratecontroller (pump) 103, a reference electrode 104, and a pressure gauge105, and is configured such that the anode solution circulates in theanode solution flow path 112 a. The anode solution tank 102 is connectedto a gas component collection unit, not illustrated, which collectsoxygen (O₂) and other gas components contained in the circulating anodesolution. The anode solution is introduced into the anode solution flowpath 112 a with the flow rate and pressure controlled in the pressurecontroller 101 and flow rate controller 103.

The CO₂ gas is supplied to the cathode gas flow path 122 a from the gassupply system 300. The gas supply system 300 has a CO₂ gas cylinder 301,a flow rate controller 302, a pressure gauge 303, and a pressurecontroller 304. The CO₂ gas is introduced into the cathode gas flow path122 a with the flow rate and pressure controlled in the flow ratecontroller 302 and pressure controller 304. The gas supply system 300 isconnected to the product collection system 400 that collects products inthe gas distributed through the cathode gas flow path 122 a. The productcollection system 400 has a gas-liquid separation unit 401 and a productcollection unit 402. Reduction products such as CO and H₂ contained inthe gas distributed through the cathode gas flow path 122 a areaccumulated in the product collection unit 402 through the gas-liquidseparation unit 401.

The anode solution circulates in the anode solution flow path 112 aduring the electrolytic reaction operation as described above. Duringthe refresh operation of the electrolysis cell 10, described below, theanode solution is discharged into the waste solution collection system600 so that the anode 111 and the anode solution flow path 112 a areexposed from the anode solution.

The waste solution collection system 600 has a waste solution collectiontank 601 connected to the anode solution flow path 112 a. A wastesolution of the anode solution is collected in the waste solutioncollection tank 601 by opening and closing not-illustrated valves. Theopening and closing of the valves and other operations are collectivelycontrolled by the control system 500. The waste solution collection tank601 also functions as a collection unit for a rinse solution suppliedfrom the refresh material supply 700. Furthermore, gaseous substancessupplied from the refresh material supply 700 and containing some liquidsubstances are also collected in the waste solution collection tank 601as necessary.

The refresh material supply 700 includes a gaseous substance supplysystem 710 and a rinse solution supply system 720. The rinse solutionsupply system 720 can be omitted in some cases. The gaseous substancesupply system 710 has a gas tank 711 that serves as a supply source forgaseous substances such as air, carbon dioxide, oxygen, nitrogen, argon,and a pressure controller 712 that controls a supply pressure of thegaseous substances. The rinse solution supply system 720 has a rinsesolution tank 721 that serves as a supply source of the rinse solutionsuch as water, and a flow rate controller (pump) 722 that controls asupply flow rate, and the like of the rinse solution. The gaseoussubstance supply system 710 and the rinse solution supply system 720 areconnected to the anode solution flow path 112 a and the cathode gas flowpath 122 a through pipes. The gaseous substances and rinse solution aresupplied to the anode solution flow path 112 a and the cathode gas flowpath 122 a by opening arid closing not-illustrated valves. The openingand closing of the valves, and other operations are collectivelycontrolled by the control system 500.

A part of the reduction products accumulated in the product collectionunit 402 is sent to a reduction performance detection unit 501 of thecontrol system 500. In the reduction performance detection unit 501, aproduction amount and a proportion of each product such as CO or H₂ inthe reduction products, are detected. The detected production amount andproportion of each product are input into a data collection andcontroller 502 of the control system 500. Furthermore, the datacollection and controller 502 collects electrical data such as a cellvoltage, a cell current, a cathode potential, and an anode potential,and data such as pressure and pressure loss inside the anode solutionflow path 112 a and the cathode gas flow path 122 a as part of cellperformance of the electrolysis cell 10, and sends the data to a refreshcontroller 503.

The data collection and controller 502 is electrically connected,through bi-directional signal lines whose illustration is partiallyomitted, to the power supply 20, the pressure controller 101 and theflow rate controller 103 of the anode solution supply system 100, theflow rate controller 302 and the pressure controller 304 of the gassupply system 300, and the pressure controller 712 and the flow ratecontroller 722 of the refresh material supply 700, in addition to thereduction performance detection unit 501, and these are collectivelycontrolled. Each pipe is provided with a not-illustrated valve, and anopening/closing operation of the valve is controlled by a signal fromthe data collection and controller 502. The data collection andcontroller 502 may control operations of the above components during theelectrolysis operation, for example.

The refresh controller 503 is electrically connected, throughbi-directional signal lines whose illustration is partially omitted, tothe power supply 20, the flow rate controller 103 of the anode solutionsupply system 100, the flow rate controller 302 of the gas supply system300, and the pressure controller 712 and flow rate controller 722 of therefresh material supply 700, and these are collectively controlled. Eachpipe is provided with a not-illustrated valve, and an opening/closingoperation of the valve is controlled by a signal from the refreshcontroller 503. The refresh controller 503 may control operations of theabove components during the electrolysis operation, for example. Therefresh controller 503 and the data collection and controller 502 may beconfigured by a single controller.

An operating operation of the carbon dioxide electrolysis device 1 ofthe embodiment will be described. FIG. 9 is a flowchart to explain anoperating method example of the carbon dioxide electrolysis device 1.First, as illustrated in FIG. 9, a startup step S101 of the carbondioxide electrolysis device 1 is performed. In the startup step S101 ofthe carbon dioxide electrolysis device 1, the following operations areperformed. In the anode solution supply system 100, the anode solutionis introduced into the anode solution flow path 112 a after its flowrate and pressure are controlled by the pressure controller 101 and theflow rate controller 103. In the gas supply system 300, the CO₂ gas isintroduced into the cathode gas flow path 122 a after its flow rate andpressure are controlled by the flow rate controller 302 and the pressurecontroller 304.

Next, a CO₂ electrolysis operation step S102 is performed. In the CO₂electrolysis operation step S102, application of an electrolysis voltageby the power supply 20 of the electrolysis device 1 that has beensubjected to the startup step S101 is started, and a current is suppliedby applying a voltage between the anode 111 and the cathode 121. Whenthe current is applied between the anode 111 and the cathode 121, anoxidation reaction near the anode 111 and a reduction reaction near thecathode 121 occur, which will be described below. The explanation of theoxidation and reduction reactions in the first embodiment can be used asappropriate.

The electrolysis operation may cause salts to precipitate in the cathodegas flow path 122 a, resulting in decreased cell performance. This isbecause ions move between the anode 111 and the cathode 121 through theseparator 30 and the ion-exchange membrane, and the ions react with gascomponents. For example, when a potassium hydroxide solution is used asthe anode solution and the carbon dioxide gas is used as the cathodegas, potassium ions move from the anode 111 to the cathode 121 and reactwith carbon dioxide to produce salts such as potassium hydrogencarbonate and potassium carbonate. When the salts are solubility or lessin the cathode gas flow path 122 a, the salts precipitate in the cathodegas flow path 122 a. The salt precipitation prevents uniform gas flowthroughout the cell and decreases the cell performance. The decrease inthe cell performance is particularly noticeable when a plurality ofcathode gas flow paths 122 a are provided. In some cases, theperformance of the cell itself can be improved by partially increasing agas flow rate. This is because the cell performance is improved byincreasing a gas pressure, which increases gas components and the likesupplied to a catalyst, or by increasing gas diffusibility. A step S103that determines whether the cell performance meets request criteria isperformed to detect such a decrease in the cell performance.

As mentioned above, the data collection and controller 502 periodicallyor continuously collects, for example, the production amount and theproportion of each product, the cell performance such as the cellvoltage, the cell current, the cathode potential, and the anodepotential of the electrolysis cell 10, the pressure inside the anodesolution flow path 112 a, the pressure inside the cathode gas flow path122 a, and the like. In addition, the data collection and controller 502has predetermined request criteria for the cell performance, and it isdetermined whether the collected data meets the set request criteria.When the collected data meets the set request criteria, the CO₂electrolysis operation is continued without stopping the CO₂electrolysis (S104). When the collected data does not meet the setrequest criteria, a refresh operation step S105 is performed.

The cell performance collected by the data collection and controller 502is defined by parameters such as, for example, an upper limit value ofthe cell voltage when a constant current is applied to the electrolysiscell 10, a lower limit value of the cell current when a constant voltageis applied to the electrolysis cell 10, and Faradaic efficiency of thecarbon compounds produced by the reduction reaction of CO₂. Here, theFaradaic efficiency is defined as a proportion of a current thatcontributed to the production of the desired carbon compound to a totalcurrent that flowed in the electrolysis cell 10. To maintain theelectrolysis efficiency, it is recommended that the refresh operationstep S105 be performed when the upper limit value of the cell voltagereaches 150% or more, preferably 120% or more of the set value when theconstant current is applied. The refresh operation step S105 may beperformed when the lower limit value of the cell current when theconstant voltage is applied reaches 50% or less, preferably 80% or less,of the set value. To maintain the production amount of the reductionproducts such as the carbon compounds, it is recommended that therefresh operation step S105 be performed when the Faradaic efficiency ofthe carbon compounds reaches 50% or less, preferably 80% or less, of theset value.

The cell performance is determined as not meeting the request criteriawhen at least one of the following parameters, for example, the cellvoltage, the cell current, the Faradaic efficiency of the carboncompounds, the pressure inside the anode solution flow path 112 a, andthe pressure inside the cathode gas flow path 122 a, does not meet therequest criteria, and the refresh operation step S105 is performed. Twoor more of the above parameters may be combined to set the requestcriteria for the cell performance. For example, the refresh operationstep S105 may be performed when both the cell voltage and the Faradaicefficiency of the carbon compounds do not meet the request criteria. Therefresh operation step S105 is performed when at least one of the cellperformances does not meet the request criteria. The refresh operationstep S105 is preferably performed at intervals of, for example, one houror more to stably perform the CO₂ electrolysis operation step S102.

When the electrolysis cell 10 mainly produces CO, for example, it can bedetermined that the request criteria of the cell performance are not metwhen hydrogen increases to at least 2 times, preferably 1.5 times ormore of a normal level. For example, in the case of CO, it can bedetermined that the request criteria of the cell performance are not metwhen CO decreases to at least 0.8 times or less, preferably 0.9 times orless, of a normal level.

When salts are detected, the salts are discharged by the rinse solution.However, when a mass transfer amount does not change evert after thesalts are discharged, it may be determined that a leak has occurred inthe electrolysis cell 10. The leak in the electrolysis cell 10 is notlimited to a gas leak between the anode 111 and the cathode 121, butincludes, for example, a gas leak from between the cathode 121 and thecathode gas flow path 122 a. This gas leak is likely to occur, forexample, when the electrolysis cell 10 with salt precipitated isoperated for a long time under conditions of high pressure in thecathode gas flow path 122 a.

FIG. 10 is a flowchart to explain an operation example of the refreshoperation step S105. First, the application of the electrolysis voltageby the power supply 20 is stopped to stop the reduction reaction of CO₂(S201). At this time, the application of the electrolysis voltage doesnot necessarily have to be stopped. Next, the supply of the gas to thecathode gas flow path 122 a is stopped, the supply of the anode solutionto the anode solution flow path 112 a is stopped, and the anode solutionis discharged from the anode solution flow path 112 a (S202). Next, therinse solution is supplied (S203) to the anode solution flow path 112 aand the cathode gas flow path 122 a for cleaning.

A refresh voltage may be applied between the anode 111 and the cathode121 while the rinse solution is being supplied. This can remove ions andimpurities attached to the cathode catalyst layer. When the refreshvoltage is applied so that the process is mainly an oxidation process,ions, impurities such as organic matters on a catalyst surface areoxidized and removed. In addition to refreshing the catalyst, ionssubstituted in an ion exchange resin when the ion-exchange membrane isused as the separator 30 can be removed by performing this process inthe rinse solution.

The refresh voltage is preferably −2.5 V or more and 2.5 V or less, forexample. Since energy is used for the refresh operation, the refreshvoltage range is preferably as narrow as possible, and more preferablybetween −1.5 V or more and 1.5 V or less, for example. The refreshvoltage may be applied cyclically so that the oxidation process and thereduction process of ions and impurities are alternately performed. Thiscan accelerate a regeneration of the ion exchange resin and thecatalyst. A voltage of a value equivalent to the electrolysis voltageduring the electrolysis operation may be applied as the refresh voltageto perform the refresh operation. In this case, a configuration of thepower supply 20 can be simplified.

Next, the gas is supplied to the anode solution flow path 112 a and thecathode gas flow path 122 a (S204) to dry the cathode 121 and the anode111. Supplying the rinse solution to the anode solution flow path 112 aand the cathode gas flow path 122 a increases a water saturation levelin the gas diffusion layer and causes a decrease in output due to gasdiffusibility. By supplying the gas, the water saturation level islowered, thus recovering the cell performance and increasing arefreshing effect. The gas is preferably supplied immediately after thedistribution of the rinse solution, or at least within 5 minutes afterthe end of the rinse solution supply. This is because the decrease inthe output due to the increase in the water saturation level issignificant. For example, when the refresh operation is performed everyhour, the output during a 5-minute refresh operation may be 0 V orsignificantly less, resulting in a loss of 5/60 of the output.

After the refresh operation is completed, the anode solution isintroduced into the anode solution flow path 112 a and the CO₂ gas isintroduced into the cathode gas flow path 122 a (S205). Then, theapplication of the electrolysis voltage between the anode 111 and thecathode 121 by the power supply 20 is resumed to restart the CO₂electrolysis operation (S206). When the application of the electrolysisvoltage has not been stopped in S201, the restart operation is notperformed. The gas may be used, or the rinse solution may be used todischarge the anode solution from the anode solution flow path 112 a.

The supply and flow of the rinse solution (S203) are performed toprevent precipitation of electrolytes contained in the anode solutionand to clean the cathode 121, the anode 111, the anode solution flowpath 112 a, and the cathode gas flow path 122 a. Therefore, water ispreferable as the rinse solution, further, water with the electricalconductivity of 1 mS/m or less is more preferable, and water with anelectrical conductivity of 0.1 mS/m or less is even more preferable. Anacid rinse solution such as low-concentration sulfuric acid, nitricacid, hydrochloric acid may be supplied to remove the electrolytes andother precipitates at the cathode 121 and the anode 111, and the like,and the electrolytes may be thereby dissolved. When thelow-concentration acid rinse solution is used, a step of supplying awater rinse solution is performed in a subsequent step. Immediatelybefore the gas supply step, the water rinse solution supply step ispreferably performed to prevent additives contained in the rinsesolution from remaining. FIG. 8 illustrates the rinse solution supplysystem 720 having one rinse solution tank 721, but when a plurality ofrinse solutions are used, such as the water and acid rinse solutions, aplurality of rinse solution tanks 721 are used accordingly.

An acid or alkaline rinse solution is particularly desirable forrefreshing the ion exchange resin. This has the effect of dischargingcations and anions that have been substituted for protons and OH⁻ in theion exchange resin. For this reason, it is preferable to alternatelydistribute the acid and alkaline rinse solutions, to combine the rinsesolution with water having the electrical conductivity of 1 mS/m orless, and to supply the gas in between the supply of the plurality ofrinse solutions to prevent mixing of the rinse solutions.

The gas used for the gas supply and flow step S204 preferably containsat least one of air, carbon dioxide, oxygen, nitrogen, and argon.Furthermore, it is preferable to use the gas with low chemicalreactivity. In this regard, air, nitrogen, and argon are preferablyused. Nitrogen and argon are even more preferable. The rinse solutionand gas for refreshing is not limited to be supplied to only the anodesolution flow path 112 a and the cathode gas flow path 122 a, but alsoto the cathode gas flow path 122 a to clean a surface of the cathode 121in contact with the cathode gas flow path 122 a. It is effective tosupply the gas to the cathode gas flow path 122 a to dry the cathode 121from the surface side in contact with the cathode gas flow path 122 a aswell.

The above describes the case where the rinse solution and gas forrefreshing are supplied to both the anode part 11 and the cathode part12, but the rinse solution and gas for refreshing can be supplied to thecathode part 12 only.

As mentioned above, it is determined whether the CO₂ electrolysisoperation step S102 should be continued or the refresh operation stepS105 should be performed in accordance with whether the cell performanceof the electrolysis cell 10 meets the request criteria. Supplying therinse solution and gas for refreshing in the refresh operation step S105prevents uneven distribution of ions and residual gas near the anode 111and cathode 121, which are factors in the deterioration of the cellperformance, and removes the electrolyte precipitation, and the like inthe cathode 121, the anode 111, the anode solution flow path 112 a, andthe cathode gas flow path 122 a. Therefore, the cell performance of theelectrolysis cell 10 can be recovered by restarting the CO₂ electrolysisoperation step S102. after the refresh operation step S105. The CO₂electrolysis performance by the electrolysis device 1 can be maintainedfor a long time by repeating the CO₂ electrolysis operation step S102and the refresh operation step S105 in accordance with the requestcriteria of the cell performance.

As mentioned above, in the carbon dioxide electrolysis device of thisembodiment, the refresh operation of the electrolysis cell is performedby temporarily flowing the rinse solution through the flow path whensalts precipitate, which can prevent the blockage of the flow path.Therefore, the decrease in the electrolysis efficiency of the carbondioxide electrolysis device can be prevented.

When performing the refresh operation, the region 122 a 1 with thehydrophilic inner wall surface 246 and the region 122 a 2 with thewater-repellent inner wall surface 247 are formed in the cathode gasflow path 122 a as illustrated in FIG. 7, which enables the rinsesolution to easily bypass the salts and flow through the region 122 a 2,which is near the salts and therefore the salts are easily dissolved,because the hydrophilic inner wall surface 246 is near the salt even ifthe salts precipitate. In addition, the rinse solution flows into theregion 122 a 2 until the salts are dissolved and can be supplied to theentire surface of the cathode 121, thus efficiently removing the salts.

EXAMPLES Example 1

A carbon dioxide electrolysis device was fabricated as follows. Iridiumoxide was formed on a surface of titanium mesh as an oxidation catalyst.Carbon paper with a catalyst layer was fabricated by spraying carbon,which supports 10.2% by mass gold on the carbon paper with MPL. Amembrane electrode composite (catalyst area 4 cm square) was prepared bylaminating this carbon paper and titanium mesh with iridium oxidesandwiched by ion-exchange membranes.

A cathode gas flow path and an anode solution flow path were formed oftitanium and were each a serpentine-shaped flow path containing twoparallel-connected flow path parts, with a land width of 0.8 mm, a flowpath width W of 1 mm, and a flow path depth h of 3 mm. An aspect ratiowas 3, and a fluid mean depth M was 0.38. An electrolysis cell wasassembled by sandwiching the membrane electrode composite between theanode solution flow path and the cathode gas flow path.

A 0.1 M potassium hydrogen carbonate solution was supplied to the anodesolution flow path at 10 mL/min as an electrolytic solution. Carbondioxide gas was supplied to the cathode gas flow path at a flow rate of320 ccm. A current was passed between the anode and the cathode with astepwise increase in a current value, and gas generated from a cathodeside was collected to measure its flow rate and conversion efficiency ofcarbon dioxide. The gas generated was sampled and identified andquantified by gas chromatography.

The current value at this time was measured with an ammeter. A partialcurrent density of carbon monoxide (CO), which is an indicator of thepercentage used for carbon monoxide production out of a total currentdensity flowed, was found from a conversion efficiency from carbondioxide to carbon monoxide. Furthermore, a utilization ratio of carbondioxide at the cathode was found from the conversion efficiency fromcarbon dioxide to carbon monoxide and a flow rate of the gas generatedfrom the cathode side. A relationship between the partial currentdensity of carbon monoxide and the utilization ratio of carbon dioxideat the cathode was evaluated in accordance with the above.

Comparative Example 1

In the carbon dioxide electrolysis device of Example 1, the electrolysiscell was assembled in the same way as in Example 1, except that thecathode gas flow path was made with the land width of 0.8 mm, the flowpath width W of 1 mm, and the flow path depth h of 0.5 mm. The cathodegas flow path had the aspect ratio of 0.5 and the fluid mean depth M of0.17. The relationship between the partial current density of carbonmonoxide and the utilization ratio of carbon dioxide at the cathode wasevaluated as in Example 1.

Comparative Example 2

In the carbon dioxide electrolysis device of Example 1, the electrolysiscell was assembled in the same way as in Example 1, except that thecathode gas flow path was made with the land width of 0.8 mm, the flowpath width W of 1 mm, and the flow path depth h of 1 mm. The cathode gasflow path had the aspect ratio of 1 and the fluid mean depth of 0.25.The relationship between the partial current density of carbon monoxideand the utilization ratio of carbon dioxide at the cathode was evaluatedas in Example 1.

Comparative Example 3

In the carbon dioxide electrolysis device of Example 1, the electrolysiscell was assembled in the same way as in Example 1, except that thecathode gas flow path was made with the land width of 0.8 mm, the flowpath width W of 1 mm, and the flow path depth h of 2 mm. The cathode gasflow path had the aspect ratio of 2 and the fluid mean depth of 0.33.The relationship between the partial current density of carbon monoxideand the utilization ratio of carbon dioxide at the cathode was evaluatedas in Example 1.

Comparative Example 4

In the carbon dioxide electrolysis device of Example 1, the electrolysiscell was assembled in the same way as in Example 1, except that thecathode gas flow path was made with the land width of 0.49 mm, the flowpath width W of 0.49 mm, and the flow path depth h of 1 mm. The cathodegas flow path had the aspect ratio of 2 and the fluid mean depth of0.16. The relationship between the partial current density of carbonmonoxide and the utilization ratio of carbon dioxide at the cathode wasevaluated as in Example 1.

FIG. 11 presents the relationships between the partial current densityof carbon monoxide and the utilization ratio of carbon dioxide at thecathode in Example 1, Comparative Example 1, Comparative Example 2, andComparative Example 3.

FIG. 12 presents the relationships between the partial current densityof carbon monoxide and the utilization ratio of carbon dioxide at thecathode in Comparative Example 3 and Comparative Example 4.

From FIG. 11, it can be seen that when the aspect ratio of the cathodegas flow path is greater than 1 and 3 or less, and the fluid mean depthM and depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4,a high CO₂ utilization ratio of 30% or more can be achieved at a high COpartial current density of 400 mA/cm² or more. It can also be seen fromFIG. 12 that even if the aspect ratios are the same, the larger thefluid mean depth M is, the higher the CO₂ utilization ratio can beobtained at the high the CO partial current density.

The above embodiments have been presented by way of example only, andare not intended to limit the scope of the inventions. Indeed, thoseembodiments may be embodied in a variety of other forms; furthermore,various omissions, substitutions, and changes in the form of theembodiments described herein may be made without departing from thespirit of the inventions. The accompanying claims and their equivalentsare intended to cover such forms or modifications as would fall withinthe scope and spirit of the inventions.

What is claimed is:
 1. A carbon dioxide electrolysis device, comprising: a cathode configured to reduce carbon dioxide and thus form a carbon compound; an anode configured to oxidize water and thus generate oxygen; a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide; an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and a separator provided between the anode and the cathode, wherein an aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, the aspect ratio being defined by a ratio of a depth of the cathode gas flow path to a width of the cathode gas flow path, and in a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4, the fluid mean depth M being defined by a ratio of a circumferential length of the cathode gas flow path to a cross-sectional area of the cathode gas flow path.
 2. The device according to claim 1, wherein the cathode gas flow path includes: a first region facing on the cathode; and a second region provided between the first region and an inner bottom surface of the cathode gas flow path, wherein a width of the second region is wider than a width of the first region.
 3. The device according to claim 1, wherein the cathode gas flow path includes: a first region facing on the cathode and having a hydrophilic first inner wall surface; and a second region provided between the first region and an inner bottom surface of the cathode gas flow path and having a water-repellent second inner wall surface.
 4. The device according to claim 1, wherein the electrolytic solution contains a metal ion.
 5. The device according to claim 1, wherein the cathode contains at least one catalyst selected from the group consisting of copper, gold, and silver.
 6. The device according to claim 1, further comprising: an electrolysis cell including the cathode, the anode, the cathode gas flow path, the anode solution flow path, and the separator; a gas supply configured to supply the gas to the cathode gas flow path; a solution supply configured to supply the electrolytic solution to the anode solution flow path; a power supply configured to apply a voltage between the anode and the cathode; a refresh material supply including a solution supply source configured to supply a rinse solution to the cathode gas flow path; and a controller configured to control operations of stopping the supply of the gas by the gas supply, stopping the supply of the electrolytic solution by the solution supply, and supplying the rinse solution to the cathode by the refresh material supply in accordance with request criteria of performance of the electrolysis cell.
 7. A method of operating a carbon dioxide electrolysis device, the carbon dioxide electrolysis device including: a cathode configured to reduce carbon dioxide and thus form a carbon compound; an anode configured to oxidize water and thus generate oxygen; a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide; an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and a separator provided between the anode and the cathode, wherein an aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, the aspect ratio being defined by a ratio of a depth of the cathode gas flow path to a width of the cathode gas flow path, and in a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4, the fluid mean depth M being defined by a ratio of a circumferential length of the cathode gas flow path to a cross-sectional area of the cathode gas flow path, the method comprising: supplying gas containing carbon dioxide to the cathode gas flow path and supplying an electrolytic solution to the anode solution flow path; applying a voltage between the anode and the cathode to reduce carbon dioxide near the cathode of the electrolysis cell to form a carbon compound and to oxidize water or hydroxide ions near the anode to generate oxygen; and stopping the supply of the gas and the electrolytic solution and supplying a rinse solution to the cathode gas flow path, in accordance with request criteria of performance of the electrolysis cell. 